To save the bandwidth for transmitting and storing speech and audio signals, the speech and audio coding technologies are applied widely. Currently, these coding technologies are mainly classified into lossy coding and lossless coding technologies.
Linear prediction (LP) analysis is widely applied in lossless compression coding to reduce the dynamic range of input signals and to remove the redundancy of the near sample points of signals, but bandwidth expansion is not generally applied in lossless coding.
In G.729 which is a lossy coding, a bandwidth expansion technology is applied by multiplying the autocorrelation coefficients with a lag-window. A 60 Hz bandwidth expansion is performed before calculating the LP coefficients by a Levinson-Durbin algorithm, with a view to making the LP analysis more stable. The steps of calculating the LP coefficients in the prior art are as follows:
1. Multiply input signals by a window function, and calculate the autocorrelation coefficients: r(0), r(1) . . . r(p), where p is the order of LP.
2. Calculate the weighting factor winlag of the autocorrelation coefficients:
                    win        lag            ⁡              (        k        )              =                            exp          ⁡                      [                                          -                                  1                  2                                            ⁢                                                (                                                            2                      ⁢                      π                      ⁢                                                                                          ⁢                                              f                        0                                            ⁢                      k                                                              f                      s                                                        )                                2                                      ]                          ⁢                                  ⁢        k            =      1        ,  …  ⁢          ,  p  ,                where f0 is a constant such as f0=60 Hz; fs is a signal sampling frequency such as 8000 Hz; and p is the order (such as 10) of LP analysis.        
3. Determine that the white-noise correction factor is winlag(0)=1.0001.
4. Calculate the adjusted autocorrelation coefficients:r(0)′=winlag(0)r(0)r(k)′=winlag(k)r(k) k=1, . . . , p 
5. Use the adjusted autocorrelation coefficients to calculate the new LP coefficients through a Levinson-Durbin algorithm.
In the prior art, every frame signal is processed in the same way.
LP analysis is widely applied in lossless coding to reduce the dynamic range of input signals and to remove the redundancy of the near sample points of signals.
In the process of implementing the present invention, the inventor finds at least these defects in the prior art: Because all signals are processed in the same way, ill-conditioned case may occur for some special input signals, and the solving of the autocorrelation matrix is instable, which leads to low compression efficiency of a lossless coder and low quality of reconstructed speech signals of a lossy coder.